Rheological properties of guayule rubber

An experimental comparative study of the rheological characterization of unvulcanized cis-1,4 polyisoprenes including Guayule, Hevea, and two synthetic polymers and their compounds is presented in this thesis. Stress relaxation, shear viscosity, elongational viscosity, and frictional force under controlled pressurized conditions are considered.

The Hevea exhibits considerably larger zero shear viscosity and maximum relaxation time than the Guayule. The Guayule's characteristics resembled those of a synthetic Ziegler based polyisoprene. For maximum relaxation times at 100°C, Hevea is 1790 seconds, Guayule 367 seconds, and the Ziegler polyisoprene 289 seconds.

The rheological properties of the polyisoprenes appear to be determined by levels of gel and long chain branching and do not correlate with molecular weight distribution.

A comparison of experimental data and theoretical predictions of single integral rheological models leads to the conclusion that the Bogue-White model offers a satisfactory representation of the behavior of unfilled polyisoprenes.

Carbon black compounds exhibit yield stresses in steady state shear flow. The steady state shear flow of compounds is interpreted in terms of a plastic viscoelastic rheological model recently developed. The agreement between experimental data and theoretical predictions is satisfactory.